This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-173014, filed Jun. 18, 1999; and No. 2000-160191, filed May 30, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a voltage-controlled oscillator capable of changing an oscillation frequency according to a control voltage or a control current, and a phase synchronization circuit and a signal processing circuit using the voltage-controlled oscillator. The invention relates to a voltage-controlled oscillator, a phase synchronization circuit and a signal processing circuit that are used, for example, in an information processing field such as a communication device, a hard disk driving unit, and an information storage device like an optical disk.
In a communication device or an information storage device, a phase synchronization circuit (or a phase-locked loop; PLL) has been used for extracting a clock signal to discriminate received data or reproduced data. As one of basic construction blocks of this PLL, there has been used a voltage-controlled oscillator (VCO) that has a function of changing an oscillation frequency according to a control voltage or a control current. It is general that a PLL including a VCO is constructed on the same LSI chip by combining the PLL with other functional block like an equalizer.
As a conventional VCO that is used in an information storage device, a multi-vibrator type circuit utilizing a charge and discharge to and from a capacitor has been used. The oscillation frequency of the multi-vibrator type VCO depends on three parameters including a capacity of the capacitor, charge and discharge currents of the capacitor, and a voltage between both terminals of the capacitor. Usually, in many cases, the oscillation frequency is controlled based on either the voltage between both terminals of the capacitor or the charge and discharge currents, with the capacity of the capacitor set constant.
According to the conventional multi-vibrator type VCO, the oscillation frequency of the VCO is controlled based on the voltage. Circuit noise attributable to voltage can be easily mixed into a control input voltage. Therefore, the oscillation frequency is easily affected by the noise due to voltage. As a result, there has been a problem that the oscillation frequency of the VCO is unstable against crosstalk noise, and it is difficult to achieve a stable oscillation. Further, there has been a problem that the oscillation frequency is also easily affected by crosstalk from other circuit blocks constructed within the same LSI chip, which makes the oscillation frequency unstable.
The above problems lead to an extreme aggravation of the performance of the device using a PLL including the conventional multi-vibrator type VCO. For example, an error rate is worsened in the information storage device.
On the other hand, as shown in FIG. 1, there has been an analog VCO consisting of a filter 301 and an automatic gain control (AGC) circuit 302. The AGC circuit 302 provides a positive feedback of an output signal of the filter 301 to an input side of the filter by controlling the output signal at a constant level. This VCO is disclosed, for example, in Japanese Patent Application Laid-open Publication No. 9-326636. The analog-type VCO using the filter has characteristics that the VCO is not easily affected by noise due to voltage, and is not easily affected by crosstalk noise from other circuit blocks.
However, the analog-type VCO using the filter has a problem in that the AGC circuit 302 has a complex construction. Further, the operation frequency area is limited to a low level based on the construction of the filter 301 and a amplitude detector 303 for detecting an output level of the AGC circuit 302. For example, an envelope-detection type amplitude detector as disclosed in Japanese Patent Application Laid-open Publication No. 9-326636 cannot accurately detect a signal in a high-frequency area, particularly, a signal exceeding a few hundred MHz. Therefore, a VCO using this amplitude detector cannot stably oscillate a signal exceeding a few hundred MHz. The data transfer speed of a device using a VCO has been increasing in recent years, and it is difficult to meet future demand for such high transfer-speed based on the conventional VCO.
As explained above, the analog-type VCO using the filter has the problems that the AGC circuit has a complex construction, and that the operation frequency area is limited to a low level by the construction of the filter and the amplitude detector of the AGC circuit.
In recent years, it is general that a waveform equalization filter is constructed on the same integrated circuit chip as that of a preamplifier and a PLL. When a filter is constructed on the integrated circuit chip, the characteristics of the chip are determined by the capacity of the capacitor constructed on the integrated circuit chip and the volume of a current flown through the capacitor.
In this case, the capacity of the capacitor constructed on the integrated circuit chip has a variance of plus minus 15%, which leads to a fluctuation in the frequency characteristics of the waveform equalization filter. As a result, equalization errors occur. Therefore, it has been desired to realize a filter having a frequency characteristic compensation function with high precision of frequency characteristics.
Consider a case where an active filter used for equalizing a reproduction signal and a PLL are structured together on the same LSI chip in a signal regenerating unit such as, for example, a hard disk driving unit, an optical disk driving unit, a data tape driving unit, etc.
When a signal reproduction speed of a signal generator has changed, it is usually necessary to optimize the cutoff frequency of the active filter according to this change. Further, it is also necessary to change the oscillation frequency and response characteristics of the PLL according to the change in the signal reproduction speed.
As described above, when the conventional PLL and the active filter for equalizing a reproduction signal are structured together on the same LSI chip, the active filter and the PLL are adjusted independent of each other. Therefore, these adjustments have had a problem of extremely lowering the productivity in the manufacturing process.
It is, therefore, an object of the present invention to provide a voltage-controlled oscillator that can be easily realized in an integrated circuit in a simple construction without an AGC circuit, and that makes it possible to supply a low-cost product. This object is to be achieved by arranging such that a delay-time variable filter controls a delay time according to a control signal input, and an output signal of the delay-time variable filter is changed into a binary pulse signal, and this signal is positively fed back to the input side of the delay-time variable filter at a desired level.
It is another object of the present invention to provide a voltage-controlled oscillator that can stably oscillate a signal in excess of a few hundred MHz, that can be easily realized in an integrated circuit, and that makes it possible to supply a low-cost product by inventive constructions of a filter and an amplitude detector of an AGC circuit. This object is to be achieved by a system in which a delay-time variable filter controls a delay time according to a control signal input, and an output signal of the delay-time variable filter is controlled at a constant level by an AGC circuit and is positively fed back to the input side of the delay-time variable filter.
It is still another object of the present invention to provide a phase synchronization circuit capable of generating a clock signal of high quality in a stable operation.
It is still another object of the present invention to provide a signal processing circuit capable of canceling aggravation in the characteristics of an active filter due to a variance in the capacity of capacitors. This object is to be achieved by linking the phase synchronization circuit to the active filter and by using a control voltage input of a voltage-controlled oscillator in a phase synchronization circuit as a control signal for setting the cutoff frequency of the active filter.
In order to achieve the above objects, according to a first aspect of the present invention, there is provided a voltage-controlled oscillator comprising: a delay-time variable filter having a signal input node, a signal output node and a control signal input node, a signal delay time between the signal input node and the signal output node being controlled according to a control signal input from the control signal input node; and a feedback loop circuit configured to receive an output signal from the delay-time variable filter and provide a positive feedback of a binary pulse signal of the output signal to the signal input node of the delay-time variable filter at a desired level.
According to a second aspect of the invention, there is provided a voltage-controlled oscillator comprising: a delay-time variable filter having a signal input node, a signal output node and a control signal input node, a signal delay time between the signal input node and the signal output node being controlled according to a control signal input from the control signal input node; a variable gain amplifier having a gain variable control signal input node, and having an amplification gain according to a gain variable control signal input from the gain variable control signal input node, configured to receive an output signal of the delay-time variable filter, amplify this received output signal, and provide a positive feedback of the amplified signal to the signal input node of the delay-time variable filter; and a sinusoidal wave detection-type amplitude detector configured to receive an amplitude corresponding to an output signal of the variable gain amplifier, detect the received amplitude, generate a negative feedback signal for controlling the amplitude corresponding to the output signal of the variable gain amplifier so that the amplitude substantially becomes a desired constant value, and supply the negative feedback signal to the gain variable control signal input node of the variable gain amplifier as the gain variable control signal.
Further, according to a third aspect of the invention, there is provided a phase synchronization circuit comprising: a delay-time variable filter having a first signal input node, a first signal output node and a first control signal input node, a signal delay time between the first signal input node and the first signal output node being controlled according to a first control signal input from the first control signal input node; a feedback loop circuit configured to receive an output signal from the delay-time variable filter, provide a positive feedback of a binary pulse signal of the output signal to the first signal input node of the delay-time variable filter at a desired level, and output the pulse signal as a clock signal; a phase comparator having a second signal input node and a third signal input node, configured to receive a reference clock signal in the second signal input node and receive the clock signal output from the feedback loop circuit in the third signal input node; a charge pump circuit configured to receive an output of the phase comparator; a loop filter configured to receive an output of the charge pump circuit, generate the first control signal according to the output of the charge pump circuit, and provide the generated first control signal to the first control signal input node; and an active filter having a fourth input signal node and a second control signal input node, configured to receive a signal to be processed in the fourth input signal node, and receive an output of the loop filter in the second control signal input node.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.